Some of the Oldest Galaxies in the Universe Orbit the Milky Way

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When the universe was young, there were no stars, galaxies, or even atoms. As space expanded, hydrogen atoms formed and coalesced into stars. Eventually, the first galaxies emerged, and some of those galaxies may still exist. New data from researchers at Durham University in the UK and Harvard in the US suggests several of those ancient galaxies could be in our own backyard. This discovery also lends credence to a leading model of the early universe.

Our current understanding holds that the early universe was not even transparent to radiation. It took thousands of years for that to change, and the first atoms of hydrogen formed about 380,000 years after the Big Bang. This period is sometimes called the Cosmic Dark Ages because there were no stars or other objects producing light or other frequencies in the electromagnetic spectrum. It would take almost 100 million years longer for the universe to cool enough that the atoms could clump together to form stars that populated the very first galaxies. But then, galaxy formation stopped for about a billion years.

The intense ultraviolet radiation from those early galaxies ionized the remaining hydrogen atoms, making it harder for them to cool and form stars. That hydrogen did eventually cool and formed massive dark matter halos around galaxies. This, in turn, shielded ionized gas from radiation, allowing it to cool and form stars. Thus, galaxy formation resumed. This is a key component of the widely accepted Lambda-cold-dark-matter model, and the team behind the latest research says its results support the model.

The team found two distinct populations of dwarf galaxies orbiting our Milky Way galaxy. One group formed during the era before dark matter halos were built up. The other is more recent and brighter. The first set is over 13 billion years old — among the oldest in the universe. The brighter ones are at least several hundred million years younger. In the “old” set are Segue-1, Bootes I, Tucana II, and Ursa Major I. The team found a similar distribution among observable dwarf galaxies in orbit of M31 (Andromeda).

This distribution of older and younger galaxies are in line with the Lambda-cold-dark-matter model. It would have been impossible to analyze the older dwarf galaxies even ten years ago, but improved instruments allowed the team to determine the properties of these tiny galaxies.